This invention relates to a rechargeable battery charger and charging method, and more particularly to a battery charger and charging method that prevents thermal run-away by reducing charging current when the temperature rises in heat producing elements of the power supply circuit during rapid charging.
The temperature of heat producing elements in the power supply circuit of a battery charger performing rapid charging of a rechargeable battery can become abnormally high. Particularly when a plurality of rechargeable batteries are rapidly charged in succession, the temperature of the heat producing elements gradually increases. Heat producing elements in the power supply circuit include the output transformer, bipolar output transistors, Field Effect (FET) output transistors, and rectifying diodes etc. These heat producing elements are attached to heat sinks in heat dissipating configurations to avoid abnormal temperature rise. The heat dissipating capacity of a heat sink is designed considering the amount of heat produced and the external temperature. The temperature rise of heat producing elements in the power supply circuit can be reduced by enlarging the heat sinks to increase their dissipating capacity. However, larger heat sinks result in higher parts cost. Further, it is difficult to fit large heat sinks inside a small case. To design a low-cost compact battery charger, small heat sinks are needed.
Heat sinks for power supply heat producing elements are designed to prevent abnormal temperature rise under normal operating conditions. However, as shown in FIG. 1, the amount of heat generation can exceed the amount of heat dissipation resulting in abnormal temperature rise when rechargeable batteries are charged successively. This problem can be solved by designing larger heat sinks. But, since large heat sinks have the drawbacks noted previously, there is a limit to heat sink size.
If a battery charger is operated when the temperature of heat producing elements in the power supply circuit has risen abnormally high, thermal run-away and failure can result. To prevent this, prior art battery chargers contain a thermal protection circuit. A device such as a temperature fuse or non-resetting relay is used in the thermal protection circuit to cut-off the output circuit.
Thermal run-away due to excessive temperature rise in power supply heat producing elements can be prevented in a battery charger containing a thermal protection circuit. However, a thermal protection circuit that uses a temperature fuse or non-resetting relay has the drawback that rechargeable battery charging is interrupted and full charge cannot be attained when the power supply circuit output is cut-off due to excessive temperature rise in its heat producing elements.
To overcome this problem, a power supply can be designed with a resetting thermal protection circuit which temporarily stops charging when the temperature of the heat producing elements rises excessively and resumes charging after they have cooled to a set temperature. However, a battery charger that resets its thermal protection circuit and resumes charging has the drawback that the rechargeable battery cannot be charged while the heat producing elements are cooling and charging time becomes lengthy. Further, a battery charger which activates a thermal protection circuit also has the drawback that the rechargeable battery voltage drops from its peak value when charging is interrupted giving a false indication of full charge. A rechargeable battery which has been falsely determined to be fully charged will not be charged further even after the heat producing elements in the power supply circuit have cooled. In this case the battery cannot be fully charged.
The present invention was developed for the purpose of overcoming these drawbacks. It is thus a primary object of the present invention to provide a battery charger having a temperature sensor that can continuously charge rechargeable batteries to full charge without interruption while effectively preventing excessive temperature rise and thermal run-away in the heat producing elements of the power supply circuit.